This application claims priority to German application 100 35 755.5, filed on Jul. 22, 2000, herein incorporated by reference.
1. Field of the Invention
The present invention describes a silicone epoxy resin, its preparation, and the use in corrosion-inhibiting, heat-stable coatings.
2. Background of the Invention
Crosslinked silicone resins containing T groups are highly heat-stable and have excellent film-forming properties. For these reasons they are used, for example, as electrical insulating materials, as heat-stable coatings and paints, as protective coating materials, as performance enhancers in organic binders, and as copolymers in conjunction with organic monomers or with organic resins. As silicone combination resins from this group, silicone epoxy resins are described in a large number of publications and patents. For the preparation of these resins a large number of synthesis routes are taken.
For the preparation of silicone resins containing epoxide groups, a large number of patents describe cohydrolysis of trialkoxysilanes containing epoxide groups, such as 3-glycidyloxypropyltrialkoxysilane, for example, with organotrialkoxysilanes and/or diorganodialkoxysilanes and/or triorganomonoalkoxysilanes. An overview is given by U.S. Pat. No. 5,516,858.
A disadvantage of this synthesis route is that only a maximum of one epoxide group per Si is attached; the resultant resins possess no carbinol (COH) functionality which is capable of crosslinking with SiOR. Accordingly, it is necessary to add heat-labile crosslinkers for the crosslinking reaction. The heat-labile crosslinkers critically reduce the heat stability of the resultant anticorrosion coating.
U.S. Pat. No. 4,250,074 describes the formation of an interpenetrating polymer network (IPN) of epoxy-polyamine and polysiloxane. Owing to the incompatibility of epoxy resin and silicone resin, only a small amount of silicone resin (about 2-4% of the formulation) can be used. Corresponding formulations exhibit no heat stability at temperatures above 200xc2x0 C.
Epoxysilane formulations wherein said silanes act as adhesion promoters are described, for example, in EP 0 556 023 A1. The low concentrations in which the epoxysilanes are used do not make it possible to formulate heat-stable systems.
Other patents describe simple mixtures of silicones and epoxides for the preparation of coatings:
The Japanese patents JP 04176368 A2 and JP 04135674 A2 describe mixtures of various epoxides, OH-functional polydimethylsiloxanes, and titanates. Owing to the lack of attachment of silicone and epoxide and to the use of linear polydimethylsiloxanes, which only effect chain extension, the coatings obtained are not heat-stable. The coatings merely exhibit good resistance to boiling water in conjunction with high flexibility and weathering stability.
The Japanese patent JP 61258871 A2 describes a mixture of silicone resin and epoxy resin. By formulating with various inorganic pigments, heat-stable formulations are obtained. Since the epoxy resin is not stabilized by chemical reaction with silicones, the formulations described require very high silicone fractions in order to achieve temperature stability; the epoxide fraction in the formulation is, accordingly, only about 10% by weight. Correspondingly, the formulations are highly priced; the large silicone fraction impairs the anticorrosion effect owing to the increased water vapor permeability.
Mixtures of epoxy resins with linear polydimethylsiloxanes, as described in the Japanese patent JP 2132165 A2, achieve only corrosion-protecting but not heat-stable properties, owing to the absence of chemical attachment.
The curing of epoxides by organosilicone curing agents is described by Vasilxc3xa9va et al. in Lakokras. Mater. Ikh Primen. 4 (1967), 18-20. The high amine content of the formulation and the lack of attachment of epoxide and silicone do not permit heat stability for the corrosion-protecting formulation.
Formulations comprising glycidyl-containing trialkoxysilanes, silicone resins and epoxides are described in WO 97/19764. The application is directed to heat-deflecting coatings. The principal binder is a silicone-modified polyether. Owing to the thermolabile polyether substituents, these formulations cannot be used for heat-stable corrosion protection.
Modification of epoxy resins with silicones is described in the Japanese patent JP 52040535 A. The reaction of methoxy- or hydroxy-functional siloxanes with epoxy resins and organic acids or anhydrides is described here. The curing agent used is a reaction product of phenyl glycidyl ether, dicyanamide and benzyltriammonium chloride. Owing to the use of this curing agent, which is not heat-stable, the resultant coatings cannot be used at elevated temperatures.
JP 50153063 A describes the modification of epoxy resin with a methylphenyl silicone resin. In a second step, the transparent silicone epoxy resin is reacted with phthalic anhydride. The carboxylic acid groups formed in the reaction with phthalic anhydride have an adverse effect on the storage stability of the silicone epoxy resin. The hydrolysis-labile Si-OR bonds, in particular, are not stable on storage in the presence of acidic groups, such as carboxylic acids (xe2x80x94COOH), for example.
DT 11 29 704 and DT 954 456 describe the reaction of epoxides with silanes or with silane mixtures. A disadvantage of this process is the reaction of the trialkoxyfunctional silanes, which proceeds irreproducibly. Depending on the reaction regime, insoluble silicone gels are formed. Accordingly, only small amounts of trifunctional silanes (T units) can be used; the major fraction of the silicone is formed of difunctional units (D units). This leads to coatings which, although flexible, lack sufficient hardness.
It is an object of the present invention to provide a heat-stable silicone epoxy resin which is thermally curable even at relatively low temperatures below 200xc2x0 C. without the use of curing agents and whose coatings possess an anticorrosion effect even after long-term temperature exposure at high temperatures above 200xc2x0 C.
The present invention provides for a process for preparing silicone epoxy resins and their use for anticorrosion temperature-stable coating.
The present invention provides for a process for preparing silicone resins which comprises reacting:
I) siloxanes of the general formula
RaSi(ORxe2x80x2)bO(4xe2x88x92axe2x88x92b)/2
xe2x80x83in which
ORxe2x80x2 is an alkoxy group with primary or secondary aliphatic alcohols, preferably having from 1 to 8 carbon atoms,
R is identical or different and is an alkyl group, preferably having from 1 to 8 carbon atoms or an aromatic group, preferably having from 6 to 20 carbon atoms,
a is from 0.1 to 2.0, and
b is from 0.1 to 1.0, with
II) one or more low molecular mass polyhydric alcohols/polyols and
III) one or more resins containing epoxide groups, containing at least two 1,2-epoxide
groups per molecule, at temperatures in the range from about 100 to about 160xc2x0 C. with removal of the alcohol HORxe2x80x2 to a degree of conversion of from about 20 to about 80% and terminating the reaction by cooling to a temperature  less than about 100xc2x0 C.
A sufficient conversion may be determined, for example, by withdrawing a portion of the reaction mixture, drying it on a glass plate and determining the transparency of the coating on the glass plate. A transparent film indicates sufficient conversion. Furthermore, the conversion may be determined precisely from the amount of ethanol distilled off.
The present invention further provides for a coating produced therewith, comprising the silicone resin of the invention, and its use.
It has surprisingly been found that using one or more low molecular mass polyhydric alcohols/polyols the modification reaction of silicone resin and epoxy resin is easier and quicker to carry out. In contrast to the reaction without alcohol/polyol, it is possible in accordance with the invention to react even epoxides having average molecular masses of from about 1000 to 5000 g/mol to give transparent binders.
As low molecular mass alcohols/polyols it is possible, for example, to use linear or branched aliphatic diols, triols or tetrols, or else low molecular mass polyesterpolyols. Suitable polyols are, for example, ethylene glycol, polyethylene glycol, trimethylolethane or trimethylolpropane. Suitable polyesterpolyols possess preferably a hydroxyl functionality of from about 200 to 600 mg KOH/g polymer (i.e., OH number). A suitable polyesterpolyol is, for example, bis(2,2-dihydroxymethyl)butyl terephthalate.
The organic fraction of the silicone resins should preferably be about 50% by weight. High fractions lead to opaque, nontransparent coatings. The molecular mass may in this case be up to 1000 g/mol.
As a result of the modification with alcohols/polyols, unreacted hydroxyl groups (Cxe2x80x94OH) are maintained after the reaction with the siloxanes, and these hydroxyl groups are suitable during the baking reaction for crosslinking, with the formation of a Sixe2x80x94OC bond. Accordingly, it is unnecessary when formulating to add an additional, generally thermolabile, crosslinker.
In comparison to a reaction product without alcohols/polyols, the binders of the invention may surprisingly be baked even at temperatures below 200xc2x0 C.
The low molecular mass polyhydric alkyl alcohol or mixtures thereof with different alcohols is suitably used in concentrations of from about 1 to about 20% by weight, preferably from about 2 to about 15% by weight, based on the overall solids of the resultant silicone-modified epoxy binder. The suitable polyesterpolyols may be used in concentrations of from about 5 to about 80% by weight, based on the alcohol/polyol component.
RaSi(ORxe2x80x2)bO(4xe2x88x92axe2x88x92b)/2 is a polysiloxane resin where 0.1 less than a  less than 2.0, 0.1 less than b  less than 1.0 and a+b less than 4, R being identical or different and being an alkyl group consisting of from 1 to 8 carbon atoms or an aromatic group having from 6 to 20 carbon atoms. Examples of alkyl groups include methyl, ethyl, isopropyl, n-butyl and t-butyl. An example of an aromatic group is phenyl. Where two or more radicals R are present, therefore, R may be identical or different. Particularly preferred substituents R are methyl or phenyl or mixtures of methyl and phenyl. Rxe2x80x2 of the alkoxy group is an alkyl radical consisting of from 1 to 8 carbon atoms such as, for example, methyl or ethyl.
The preparation of silicone resins per se has long been known in the literature (see W. Noll in xe2x80x9cChemie und Technologie der Siliconexe2x80x9d [Chemistry and Technology of Silicones], Verlag Chemie, Weinheim (1968)) and is described, for example, in DE 34 12 648 C.
As epoxy resins it is possible to use commercially customary, preferably nonsilicone binders containing at least two 1,2-epoxide groups per molecule. Examples of suitable low molecular mass epoxy resins (MW less than 5000 g/mol) are commercial aliphatic epoxides (Eponex(copyright) 1513, Epodil(copyright) 757 or Epilox(copyright) M700) or aromatic epoxides (Epikote(copyright) 1001, Epikote(copyright) 1004, Epikote(copyright) 1007, Epon(copyright) 828). They are used for the modification reaction in contrast to high molecular mass epoxy resins, having for example average molecular masses above 8000 g/mol. Owing to the excessive incompatibility, it is impossible to prepare transparent binders.
In the context of the present invention, the silicone resins are prepared with particular preference by adjusting the molar ratios of COH to SiORxe2x80x2 groups to be greater than or equal to 1. Where the ratio is adjusted to be less than 1, and, accordingly, an excess of SiOH groups is obtained, the silicone resins formed are not stable on storage.
The production of heat-stable coatings even on long-term exposure to temperatures above 200xc2x0 C. generally requires silicone contents of more than 30% by weight, based on the overall binder solids. At silicone contents above 90% by weight, the corrosion protection effect is impaired owing to the increased water vapor permeability.
The components are normally reacted with one another in proportions such that one SiORxe2x80x2 group corresponds approximately to one COH group, or the ratio of the COH groups to the SiORxe2x80x2 groups is greater than 1. Marked excesses of SiORxe2x80x2 lower the storage stability; the minimum curing temperature required is increased.
Examples of suitable transesterification catalysts are metal catalysts based, for example, on magnesium, cobalt, iron, aluminum, titanium, lead, zinc or tin, in the form for example of their laurates, octoates, acetates, acetylacetonates, neodecanoates or naphthalates. For instance, use may be made of titanium esters or cobalt salts of organic acids or sulfonic acids, such as p-toluenesulfonic acid or benzenesulfonic acid. Particularly suitable organotin catalysts are, for example, dibutyltin dilaurate, dibutyltin dioctoate or dibutyltin diacetate. Particularly suitable organotitanium catalysts are, for example, tetra-n-butyl titanate or tetra-isopropyl titanate.
The modification reaction is normally conducted in one or more solvents (solvent mixture). Examples of suitable aromatic solvents are toluene or xylene; examples of suitable aliphatic solvents are esters such as methoxypropyl acetate or ketones such as cyclohexanone.
In order to stabilize the silicone-modified epoxy resin, low molecular mass aliphatic monohydric alcohols, preferably having from 1 to 20 carbon atoms, such as isobutanol, for example, may be added at the end of the reaction. Alternatively, water may also be added for stabilization.